1. Field of the Invention
The present invention relates to radiation curable compositions, and more specifically to radiation curable inks and inkjet inks, which are suitable for food packaging applications.
2. Description of the Related Art
A free radical photoinitiator initiates the polymerization of monomers when exposed to actinic radiation by the formation of a free radical. Photoinitiators are frequently used in UV-curable compositions, such as UV-curable inkjet inks.
Two types of free radical photoinitiators can be distinguished. A Norrish Type I initiator is an initiator which cleaves after excitation, yielding the initiating radical immediately. A Norrish Type II-initiator is a photoinitiator which is activated by actinic radiation and forms free radicals by hydrogen abstraction from a second compound that becomes the actual initiating free radical. This second compound is called a co-initiator or polymerization synergist.
A photoinitiator can be a monofunctional compound, but can also be a multifunctional compound, i.e. having more than one photoinitiating group. WO 03/033492 (COATES BROTHERS) discloses multifunctional thioxanthone photoinitiators.
When radiation curable compositions are used for food packaging, toys and dental applications, the amount of extractable residues is a critical issue and needs to be minimized. Low molecular weight products are usually not completely built into the polymer network and are prone to be readily extracted or to diffuse out of the cured composition.
Especially Norrish Type II initiators are a point of concern regarding extractable residues. Norrish Type II photo-initiators, such as benzophenone and thioxanthone, always require a co-initiator. Aliphatic tertiary amines, aromatic amines and thiols are preferred examples of co-initiators. After transfer of a hydrogen atom to the Norrish Type II initiator, the radical generated on the co-initiator initiates the polymerization. Theoretically the co-initiator is built into the polymer network. However, it is highly unlikely that both the hydrogen transfer and the initiation reaction yields are a hundred percent. Side reactions are likely to occur, resulting in unreacted co-initiator and side products being present in the cured composition. In food packaging printed upon with such a radiation curable composition, these low molecular weight residues remain mobile and if toxic will cause health risks upon being extracted into the food.
One approach to minimize extraction of the photoinitiator is to use Norrish Type II initiators with a higher molecular weight. However, polymeric initiators have a certain tendency to lose reactivity. Hence, often considerable amounts of polymeric initiators are required in order to reach the desired curing speed, thereby also increasing the viscosity to an undesirable level for a great number of applications using radiation curable compositions, such as e.g. inkjet printing.
EP 1674499 A (AGFA GRAPHICS) discloses radiation curable compositions and photoreactive polymers comprising a dendritic polymer core with at least one initiating functional group and at least one co-initiating functional group. While the use of a dendritic polymer core is advantageous for maintaining a low viscosity of the radiation curable composition, an improvement in curing speed is still desirable, especially in the absence of nitrogen inertisation.
Another approach in solving the extraction problem is to design a photoinitiator having one or more ethylenically unsaturated polymerizable groups so that it can be copolymerized with the other monomers of the radiation curable composition. However the copolymerization leads to a reduced mobility of the photoinitiator and hence a reduction in curing speed can be observed.
JP 2004-224993 (NIPPON KAYAKU) discloses self-photopolymerization type photopolymerization initiators for reducing its evaporation or sublimation from cured films of radiation curable compositions.
Another problem is that polymerizable Type II-initiators known in the prior art are only soluble to a limited extent in radiation curable formulations, resulting in a lower curing speed. In order to achieve sufficient curing speed, a mixture of polymerizable and non-polymerizable Type II-initiators initiators is used. For example, EBECRYL™ P36 from Cytec Surface Specialties is a polymerizable acrylated benzophenone further containing non-polymerizable benzophenone and hence considerable amounts of extractable photoinitiator and residues from cured compositions is observed.
Thus there still remains a need for Norrish Type II photoinitiators exhibiting good solubility in a broad range of radiation curable compositions, high reactivity with a low impact on the viscosity of the radiation curable composition and while still maintaining a low amount of extractable residues.